Spheroidizing treatment method for molten metal of spheroidal graphite cast iron

ABSTRACT

A graphite spheroidizing agent containing: 30-80 wt % of Si; Mg; RE (rare earth element) which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; Ca; and Al is used. The graphite spheroidizing agent is added so as to satisfy the conditions that an amount of RE equivalent to 0.001-0.009 wt % of the total weight of the molten metal, an amount of Ca equivalent to 0.001-0.02 wt % of the total weight of the molten metal, and an amount of Al equivalent to 0.001-0.02 wt % of the total weight of the molten metal are added to the molten metal, and that the molten metal contains 0.03-0.07 wt % of Mg after the graphite spheroidizing treatment. It is possible to suppress crystallization of chunky graphite in a thick section of spheroidal graphite cast iron and deterioration of mechanical properties, with a low cost.

TECHNICAL FIELD

The present invention relates to a spheroidizing treatment method for a molten metal of a spheroidal graphite cast iron.

BACKGROUND ART

Spheroidal graphite cast iron is a material, in which graphite is spheroidized in its as-cast state, and which has excellent mechanical properties (Young's modulus, tensile strength, and elongation). The spheroidization of graphite is conducted by adding a graphite spheroidizing agent in a ladle. The graphite spheroidizing agent contains Mg, rare earth (hereinafter abbreviated to “RE”), Ca, Al and so on.

In large-sized thick-walled products or thick portions of products (hereinafter they will be generically referred to as “thick sections”) in which cooling rate is low and thus eutectic solidification time is long, chunky graphite which is an abnormal graphite structure is likely to be crystallized in the metallographic structure of the spheroidal graphite cast iron. The crystallization of the chunky graphite results in significant reduction of Young's modulus, tensile strength and elongation of the cast iron material.

An example of a graphite spheroidizing agent for suppressing generation of chunky graphite is disclosed in JP2007182620A. However, JP2007182620A fails to teach a preferable amount of the graphite spheroidizing agent to be added to the molten metal.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a graphite spheroidizing treatment method by which crystallization of chunky graphite in a thick section of spheroidal graphite cast iron can be suppressed with enhanced reliability.

In a graphite spheroidizing treatment method in one embodiment of the present invention, a graphite spheroidizing agent to be used contains: 30-80 wt % of Si; Mg; RE (rare earth element) which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; Ca; and Al. Normally, the balance of the graphite spheroidizing agent (elements other than the aforementioned elements) contains iron and unavoidable impurities. The graphite spheroidizing agent may further contain S as an optional element. The graphite spheroidizing agent is added to the molten metal such that an amount of RE equivalent to 0.001-0.009 wt % of the total weight of the molten metal, an amount of Ca equivalent to 0.001-0.02 wt % of the total weight of the molten metal, and an amount of Al equivalent to 0.001-0.02 wt % of the total weight of the molten metal are added to the molten metal, and such that the molten metal contains 0.03-0.07 wt % of Mg after the graphite spheroidizing treatment.

According to the aforementioned embodiment, the contents of RE, Ca, and Al which show a graphitizing effect and accelerate crystallization of chunky graphite are lowered but optimized. Therefore, it is possible to suppress crystallization of chunky graphite in a thick section of spheroidal graphite cast iron, particularly in a thick section where the eutectic solidification time is 1.0 ks (i.e., 1000 seconds) or more.

In addition, according to the aforementioned embodiment, the amounts of Ca and Al which promote formation of slag and dross are reduced but optimized. Therefore, a clean molten metal can be obtained. As a result, a product having less defects such as slag inclusion and pinholes can be obtained.

Furthermore, according to the aforementioned embodiment, since the usage of RE that is expensive and somewhat unstable in price is reduced, material cost can be reduced and also sensitivity to price fluctuations can be lowered.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical microphotograph showing an example of the structure of a sound spheroidal graphite cast iron.

FIG. 2 is an optical microphotograph showing an example of a structure in which chunky graphite has been crystallized.

FIG. 3 is a schematic illustration of a graphite spheroidizing treatment by a sandwich method.

FIG. 4 is a schematic illustration of a graphite spheroidizing treatment by a wire feeder method.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

A graphite spheroidizing treatment method in one embodiment of the present invention will be described below.

The shape of products to which the graphite spheroidizing treatment method in this embodiment is applied is not limited. However, the graphite spheroidizing treatment method achieves an excellent structure-improving effect especially on those products having a eutectic solidification time of 1.0 ks or more, for example, in a range of 1.0-100 ks, or those products having a maximum wall thickness of 100-500 mm.

The graphite spheroidizing agent used in the embodiment of the graphite spheroidizing treatment method of the present invention contains 30-80 wt % of Si (silicon); Mg (magnesium); RE (rare earth element); Ca (calcium); and Al (aluminum). Besides, in this embodiment, the graphite spheroidizing agent is added to the molten metal so as to satisfy the following two conditions. The first condition is that the adding amounts of the following elements to the molten metal (the ratio of the weight of the added elements to the total weight of the molten metal) are RE: 0.001-0.009 wt %; Ca: 0.001-0.02 wt %; Al: 0.001-0.02 wt %. The second condition is that the molten metal contains 0.03-0.07 wt % Mg after the graphite spheroidizing treatment.

The graphite spheroidizing agent may further contain S. The parts of the graphite spheroidizing agent other than the aforementioned elements typically consist of Fe (iron) and unavoidable impurities.

While the Mg added to the molten metal takes part in nucleation of graphite, part of the Mg does not serve as a nucleus for graphite but becomes an oxide or a composite compound with the RE and so on, with the oxide or composite compound being wasted as slag. Therefore, it is preferable for Mg to be added to the molten metal in such an addition amount that after the graphite spheroidizing treatment (namely, immediately before casting into a mold) the Mg will remain in the molten metal in an amount for maintaining a graphite spheroidizing effect, specifically, in an amount of 0.03-0.07 wt %.

If the content of Mg in the molten metal after the graphite spheroidizing treatment is less than 0.03 wt %, graphite shape deteriorates due to deficiency in the deoxidizing effect of Mg and deficiency in the amount of Mg serving as a raw material for graphite nuclei. If the Mg content exceeds 0.07 wt %, on the other hand, exploded graphite is formed. In either case, the graphite shape is deteriorated, and mechanical properties are degraded accordingly. The aforementioned value for the amount of Mg is an ordinary value in the manufacture of spheroidal graphite cast iron.

The relationship between the amount of Mg added to the molten metal and the Mg content of the molten metal after the graphite spheroidizing treatment is well known to those skilled in the art. For example, where a spheroidizing agent containing 5 wt % of Mg is added to the molten metal in an amount of 1.5 wt % (namely, where Mg is added to the molten metal in an amount of 0.075%), the Mg content (analyzed value) of the molten metal after the graphite spheroidizing treatment will be 0.035-0.055 wt %. It is also well known that an increase in the amount of Mg added lowers the yield of Mg. Accordingly, it is easy for those skilled in the art to determine the addition amount of Mg to the molten metal such that Mg will remain in the molten metal in an amount of 0.03-0.07 wt % after the graphite spheroidizing treatment.

In adding RE, adding Ce (cerium) alone or adding La (lanthanum) alone is preferable, instead of adding a plurality type of RE elements in the form of an alloy or mixture of them. When the Ce alone or the La alone is added solely and in an appropriate amount, excellent mechanical properties are obtained. Where Ce alone is used as RE, the purity of the Ce is preferably 80-100 wt %. Where La alone is used as RE, the purity of the La is preferably 80-100 wt %. It should be noted, however, that the above compositional specification does not exclude a case in which, when Ce is added as the RE, the Ce contain La as an unavoidable impurity which cannot be completely separated from the Ce.

In a case where RE is used as a constituent of the graphite spherodizing agent, it is a common practice to use the RE in the form of an alloy (misch metal) in which Ce:La=2:1. In this embodiment, on the other hand, the misch metal is further subjected to refining or purification, and high-purity Ce or La is added solely.

As mentioned above, it is preferable to add 0.001-0.009 wt % of RE (Ce or La) to the molten metal. If the amount of the RE added is less than 0.001 wt %, an ability to neutralize graphite spheroidization inhibiting elements is insufficient, leading to an unfavorable graphite shape. If the RE addition amount exceeds 0.009 wt %, on the other hand, a large amount of chunky graphite is crystallized. In both cases, mechanical properties are deteriorated. For obtaining a favorable graphite shape more certainly, it is more preferable to add 0.002-0.005 wt % of RE (Ce or La) to the molten metal.

In a case where the graphite spheroidizing agent contains S and is applied to casting of a thick-walled product, the ratio of the amount of RE added to the molten metal to the amount of S (sulfur) added to the molten metal (i.e., the weight ratio of RE/S) is preferably set in the range of 0.06-1.60 (in a case where 0.005-0.030 wt % of S and 0.002-0.008 wt % of RE are added to the molten metal). According to this setting, it is possible to more reliably obtain a good graphite shape. It should be noted that it has been said that an RE/S ratio in the range of 2.0-5.0 is recommendable for obtaining a good graphite shape in a case of thin-walled products.

The molten metal of a cast iron (raw molten metal) may contain a comparatively large amount of S (it may depend on the used melting furnace or the used melting method). The S is positively added as mentioned above if the molten metal (raw molten metal) has been sufficiently desulfurized. Where S is contained in a comparatively large amount in the raw molten metal, the positive S addition may be avoided; or, alternatively, S may be added in an amount obtained by subtracting the amount of S originally contained in the raw molten metal from the necessary amount.

As mentioned above, it is preferable to add 0.001-0.020 wt % of Ca and 0.001-0.020 wt % of Al to the molten metal. If the amount of Ca or Al added is less than 0.001 wt %, nucleation of graphite does not take place sufficiently. If the amount of Ca or Al added is more than 0.020 wt %, on the other hand, crystallization of chunky graphite occurs, and slag and dross are liable to be formed. As a result, the product obtained may have such defects as slag inclusion and pinholes.

As a reference, an example of a good graphite shape appearing in a casting with a eutectic solidification time of 2.5 ks is shown in the photograph in FIG. 1, whereas an example of a bad graphite shape is shown in the photograph in FIG. 2. A lot of crystallization of chunky graphite is seen, particularly at the left, in the photograph in FIG. 2. In the example of FIG. 1, the amounts of RE, Ca, and Al added were optimized as aforementioned, whereas in the example of FIG. 2, Ca and Al were added in excessive amounts.

The graphite spheroidizing agent as above is applicable to all the known graphite spheroidizing treatment methods exemplified by, but not limited to, a sandwich method, a tundish method, and a wire feeder method.

FIG. 3 schematically illustrates the sandwich method. In the sandwich method which is often used in general, a reaction groove (pocket) formed in a bottom portion of a ladle is filled with a graphite spheroidizing agent SA, which is thoroughly covered with a covering agent CA (scrap iron, Fe—Si, etc.). Raw molten metal RM at 1400-1500° C. is poured into the ladle, and the spheroidizing treatment of graphite is effected through the reaction between the graphite spheroidizing agent and the molten metal. A larger amount of Mg addition results in a more vigorous reaction. Such a vigorous reaction may be moderated by adding Ca in a larger amount within the optimum range.

An example of a suitable composition of the graphite spheroidizing agent for carrying out the sandwich method is as follows.

Si: 30-80 wt %

Mg: 3-8 wt %

RE: 0.1-0.6 wt % (where RE is comprised of Ce with a purity level of 80-100 wt %, or La with a purity level of 80-100 wt %), preferably, 0.2-0.5 wt %

Ca: 0.1-1.3 wt %

Al: 0.1-2.0 wt %

The balance: Fe and unavoidable impurities (which may include S)

As the graphite spheroidizing agent for use in the sandwich method, for example, the followings are adopted. In a case of a small casting of 10 to 500 kg in weight, a particulate graphite spheroidizing agent with a particle diameter of 1-5 mm is preferably used so as to ensure complete melting of the agent. On the other hand, in a case of a large casting in excess of 500 kg in weight, which requires a longer time for solidification than the small castings, a lumpy graphite spheroidizing agent with a particle diameter of 5-70 mm is preferably used so as to suppress fading as much as possible.

FIG. 4 schematically illustrates the wire feeder method. With a lid of a ladle L opened, a raw molten material at 1400-1500° C. is poured into the ladle, and then the lid is closed. In this condition, a predetermined length portion of a wire W is fed into the molten metal MM by a feeder (not shown) (see the arrow in the figure). As the wire, there can be used, for example, a wire-shaped member wherein a graphite spheroidizing agent with a particle diameter of 0.1 to 1 mm is sealed in an inner cavity of a hollow sheath member made of iron.

An example of a suitable composition of the graphite spheroidizing agent (to be sealed in the wire) for carrying out the wire feeder method is as follows.

Si: 30-80 wt %

Mg: 9-25 wt %

RE: 0.3-1.8 wt % (where RE is comprised of Ce with a purity level of 80-100 wt %, or La with a purity level of 80-100 wt %)

Ca: 0.1-6.0 wt %

Al: 0.1-6.0 wt %

In this case, also, the graphite spheroidizing agent may further contain S.

Since the price of the wire per unit length is high, a graphite spheroidizing agent containing the constituent elements in high concentrations is sealed in the wire, so as to obtain the desired composition with less consumption of the wire material.

In general, the molten metal having undergone the graphite spheroidizing treatment is cast into a mold at a temperature of 1300-1400° C., whereby a thick-walled spheroidal graphite cast iron product having good mechanical properties is obtained. In the case of a thick-walled product having a eutectic solidification time longer than 1 ks, however, it is preferable to set the casting temperature to a lower value, for example, 1270-1370° C.

In the case where the aforementioned graphite spheroidizing agent is used, mechanical properties can further be improved by conducting inoculation after the graphite spheroidizing treatment. For instance, an inoculant having a composition of Fe-(30-75 wt %)Si-(0-3.0 wt %)Ca-(0-3.0 wt %)Al-(0-1.0 wt %)Ba may be used. An addition of the inoculant in an amount per run of 0.01-0.20 wt % (the weight of the inoculant/the weight of the molten metal) may be conducted one to five times. Such an inoculation is not limited to the one conducted in the ladle, before it is poured into the mold, but may be conducted during or concurrently with the pouring into the mold, as in the cases of pouring of basin inoculation and in-mold inoculation.

EXAMPLE

Now, the results of an experiment conducted for confirming the effects of the graphite spheroidizing treatment will be described below.

There was prepared a mold for casting a 100 mm-thick test piece designed so as to have a eutectic solidification time of 1.0 ks.

A particulate graphite spheroidizing agent having the following composition was prepared.

Si: 75 wt %

Mg: 5.1 wt %

Ca: 0.7 wt % or 2.0 wt %

Al: 0.7 wt % or 2.0 wt %

RE: 0.15 wt %, 0.3 wt %, 0.6 wt % or 1.3 wt %

S: 0.33 wt %, 0.67 wt % or 2.0 wt %

The balance: Fe and unavoidable impurities

By the sandwich method illustrated in FIG. 3, the aforementioned graphite spheroidizing agent was added to 30 kg of a raw molten metal. The graphite spheroidizing agent was added to the raw molten material in an amount of about 1.5 wt % so that the amounts of the constituent elements added to the molten metal, based on the weight of the molten metal, would be as follows.

Mg: 0.075 wt %

Ca: 0.01 wt % or 0.03 wt %

Al: 0.01 wt % or 0.03 wt %

RE: 0.002 wt %, 0.004 wt %, 0.008 wt % or 0.019 wt %

S: 0.005 wt %, 0.010 wt % or 0.030 wt %

When Mg was added in the aforementioned amount, the amount of Mg contained in the molten metal after the graphite spheroidizing treatment was about 0.045 wt %.

The following elements were contained in the molten metal (raw molten material).

C: 3.5-3.7%

Si: 2.4-2.6%

Mn: 0.5-0.6%

Immediately (so immediately that fading would not occur) after the graphite spheroidizing treatment, casting was conducted. It is to be noted here, however, that Samples Nos. 26-29 were subjected, after the graphite spheroidizing treatment and before the casting into a mold, to inoculation using the aforementioned inoculant in a ladle twice, and immediately (so immediately that fading would not occur) thereafter, casting was conducted. The composition values for Sample Nos. 26-29 are the values after the graphite spheroidizing treatment and before the inoculation.

After the test pieces were cast, they were processed into a predetermined shape, and subjected to a tensile test to measure tensile strength and elongation. In addition, the hardness of the test pieces was measured, and, further, microstructure observation under microscope and measurement of spheroidal graphite rate (percent nodularity) were carried out. These tests were all carried out in accordance with the requirements specified for spheroidal graphite cast iron (JIS G 5502) in the Japanese Industrial Standards.

The test results are set forth in Table 1 below.

TABLE 1 Tensile Percent RE, RE Ca Al S Strength Elongation Hardness Nodularity No. kind (%) (%) (%) (%) (MPa) (%) (HB) (%) 1 La 0.002 0.01 0.03 0.010 450 12 158 88 2 La 0.002 0.03 0.01 0.010 470 11 164 87 3 ∘ La 0.002 0.01 0.01 0.005 467 16 167 95 4 ∘ La 0.002 0.01 0.01 0.030 464 15 161 96 5 ∘ La 0.004 0.01 0.01 0.005 460 16 169 94 6 La 0.004 0.03 0.01 0.010 488 12 164 90 7 La 0.008 0.01 0.01 0.005 477 14 158 91 8 La 0.008 0.03 0.03 0.010 465 12 162 87 9 La 0.019 0.01 0.01 0.010 440 13 153 84 10 Ce + 0.002 0.01 0.03 0.010 434 11 151 88 La 11 Ce + 0.002 0.03 0.01 0.010 480 12 164 89 La 12 Ce + 0.004 0.01 0.01 0.010 505 10 171 90 La 13 Ce + 0.008 0.03 0.03 0.010 442 12 158 86 La 14 Ce + 0.008 0.01 0.03 0.010 452 13 157 89 La 15 Ce + 0.008 0.03 0.01 0.010 471 12 163 89 La 16 Ce + 0.019 0.01 0.01 0.010 433 11 152 87 La 17 Ce 0.002 0.01 0.03 0.010 484 15 169 88 18 Ce 0.002 0.03 0.01 0.010 465 12 169 89 19 ∘ Ce 0.002 0.01 0.01 0.005 460 17 156 96 20 ∘ Ce 0.002 0.01 0.01 0.030 458 20 164 97 21 ∘ Ce 0.004 0.01 0.01 0.005 458 17 167 92 22 Ce 0.004 0.03 0.01 0.010 465 11 156 85 23 ∘ Ce 0.008 0.01 0.01 0.005 445 18 164 91 24 Ce 0.008 0.03 0.03 0.010 450 12 162 84 25 Ce 0.019 0.01 0.01 0.010 450 14 160 88 26 ∘ La 0.004 0.01 0.01 0.010 459 19 163 97 27 ∘ La 0.008 0.01 0.01 0.005 452 18 164 95 28 ∘ Ce 0.004 0.01 0.01 0.010 460 20 167 98 29 ∘ Ce 0.008 0.01 0.01 0.005 458 21 162 97

The mark “∘” in the second column from the left in Table 1 is affixed to the samples which satisfy all of the following four conditions.

Condition 1—RE is Ce alone or La alone.

Condition 2—The amount of RE added is in the range of 0.001-0.009 wt %.

Condition 3—The amount of Ca added is in the range of 0.001-0.02 wt %.

Condition 4—The amount of Al added is in the range of 0.001-0.02 wt %.

All the samples marked with “∘” have the RE/S ratio in the range of 0.06-1.60.

For Sample Nos. 10-16 in Table 1, the aforementioned misch metal was used as the RE.

As shown in Table 1, though the tensile strength was partially below 450 MPa in the condition where the amount of RE after the graphite spheroidizing treatment was 0.019 wt %, the tensile strength was not less than 450 MPa in the other conditions. Even where the amount of RE was small, elongation tended to be lowered when the Ca and Al addition amounts were 0.03% and when the Ce—La alloy was used as RE.

For Sample Nos. 26-29 subjected twice to inoculation using the inoculant, a lowering in tensile strength was not observed, and a high elongation was obtained.

As reference values for quality of castings, the reference values for mechanical properties applied to cast iron products with a chief thickness of 60-200 mm in FCD 400-15A and FCD 500-7A described in Table 3 “Mechanical properties of cast-on test sample” of JIS G 5502 were taken into consideration. The reference values are a tensile strength of not less than 370 N/mm² (=370 MPa), an elongation of not less than 12%, and a hardness of HB 120-180 for FCD 400-15A. Besides, the reference values are a tensile strength of not less than 420 N/mm² (=420 MPa), an elongation of not less than 5%, and a hardness of HB 130-230 for FCD 500-7A. It is desirable for the samples to satisfy these reference values with allowance.

The samples marked with “∘” in the second column from the left in Table 1 are all high in percent nodularity and elongation, showing that good quality has been obtained. 

1. A spheroidizing treatment method for spheroidizing graphite by addition of a graphite spheroidizing agent to a molten metal, wherein: the graphite spheroidizing agent contains: 30-80 wt % of Si; Mg; RE (rare earth element) which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; Ca; and Al; and the graphite spheroidizing agent is added so as to satisfy the conditions that an amount of RE equivalent to 0.001-0.009 wt % of the total weight of the molten metal, an amount of Ca equivalent to 0.001-0.02 wt % of the total weight of the molten metal, and an amount of Al equivalent to 0.001-0.02 wt % of the total weight of the molten metal are added to the molten metal, and that the molten metal contains 0.03-0.07 wt % of Mg after the graphite spheroidizing treatment.
 2. The spheroidizing treatment method according to claim 1, wherein the graphite spheroidizing agent further contains S, and the graphite spheroidizing agent is added such that an amount of RE equivalent to 0.002-0.008 wt % of the total weight of the molten metal is added, and a ratio of the amount of RE added to the molten metal to an amount of S added to the molten metal is within a range of 0.06-1.60.
 3. The spheroidizing treatment method according to claim 1, wherein the graphite spheroidizing agent contains: 30-80 wt % of Si; 3-8 wt % of Mg; 0.1-0.6 wt % of RE which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; 0.1-1.3 wt % of Ca; and 0.1-2.0 wt % of Al, and wherein the graphite spheroidizing agent is added to the molten metal by a sandwich method.
 4. The spheroidizing treatment method according to claim 1, wherein the graphite spheroidizing agent contains: 30-80 wt % of Si; 9-25 wt % of Mg; 0.3-1.8 wt % of RE which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; 0.1-6.0 wt % of Ca; and 0.1-6.0 wt % of Al, and wherein the graphite spheroidizing agent is added to the molten metal by a wire feeder method.
 5. The spheroidizing treatment method according to claim 3, wherein the graphite spheroidizing agent has a particulate shape having a particle diameter of 1-5 mm.
 6. The spheroidizing treatment method according to claim 3, wherein the graphite spheroidizing agent has a lumpy shape having a length of 5-70 mm.
 7. The spheroidizing treatment method according to claim 4, wherein the graphite spheroidizing agent has a wire-like shape in which graphite spheroidizing agent particles having a particle diameter of 0.1-1.0 mm are included continuously.
 8. The spheroidizing treatment method according to claim 1, wherein the spheroidizing treatment is conducted at a temperature of 1400-1500° C., and the molten metal is poured into a mold at a temperature of 1270-1370° C.
 9. The spheroidizing treatment method according to claim 2, wherein the graphite spheroidizing agent contains: 30-80 wt % of Si; 3-8 wt % of Mg; 0.1-0.6 wt % of RE which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; 0.1-1.3 wt % of Ca; and 0.1-2.0 wt % of Al, and wherein the graphite spheroidizing agent is added to the molten metal by a sandwich method.
 10. The spheroidizing treatment method according to claim 2, wherein the graphite spheroidizing agent contains: 30-80 wt % of Si; 9-25 wt % of Mg; 0.3-1.8 wt % of RE which comprises Ce with a purity level of 80-100 wt % or La with a purity level of 80-100 wt %; 0.1-6.0 wt % of Ca; and 0.1-6.0 wt % of Al, and wherein the graphite spheroidizing agent is added to the molten metal by a wire feeder method.
 11. The spheroidizing treatment method according to claim 9, wherein the graphite spheroidizing agent has a particulate shape having a particle diameter of 1-5 mm.
 12. The spheroidizing treatment method according to claim 9, wherein the graphite spheroidizing agent has a lumpy shape having a length of 5-70 mm.
 13. The spheroidizing treatment method according to claim 10, wherein the graphite spheroidizing agent has a wire-like shape in which graphite spheroidizing agent particles having a particle diameter of 0.1-1.0 mm are included continuously.
 14. The spheroidizing treatment method according to claim 2, wherein the spheroidizing treatment is conducted at a temperature of 1400-1500° C., and the molten metal is poured into a mold at a temperature of 1270-1370° C. 